This invention relates to a solid electrolytic capacitor with face-down terminals having electrodes directly drawn out to its board mount side.
Solid electrolytic capacitors using tantalum, niobium, or the like as a valve-action metal are small in size, large in capacitance, and excellent in frequency characteristic and have thus been widely used in decoupling circuits or power circuits of CPUs and so on. Following the recent development of portable electronic devices, there has been increasing commercialization of solid electrolytic capacitors with face-down terminals for the purpose of achieving low ESR (equivalent series resistance) and low ESL (equivalent series inductance) particularly at a high frequency range.
As a technique disclosed as such a solid electrolytic capacitor with face-down terminals, there is an example of Patent Document 1 (Japanese Unexamined Patent Application Publication (JP-A) No. 2002-367862). FIG. 11 is a longitudinal sectional view showing a solid electrolytic capacitor with face-down terminals according to prior art 1, wherein numeral 51 denotes a capacitor element, 52 an anode lead, 53 an anode terminal, 55 a cathode terminal, and 64 a casing resin.
However, as shown in FIG. 11, the distance between the cathode terminal 55 and the anode terminal 53 at the mount surface is long and thus the distance of a current path becomes long when the capacitor is mounted on an external circuit board, and therefore, the ESR and ESL of the entire capacitor cannot be said to be minimal.
As a technique disclosed for the purpose of achieving low ESR and low ESL at a higher frequency range, there is an example of Patent Document 2 (Japanese Unexamined Patent Application Publication (JP-A) No. 2003-133177 (FIG. 15)). FIGS. 12A and 12B show a solid electrolytic capacitor with face-down terminals according to prior art 2, wherein FIG. 12A is a longitudinal sectional view of the capacitor alone and FIG. 12B is a longitudinal sectional view in which the capacitor is mounted on a circuit board. Herein, 62 denotes a circuit board, 66 a solder, and 67 a land.
As shown in FIGS. 12A and 12B, in the solid electrolytic capacitor with face-down terminals according to this technique, a cathode terminal 55 is provided so as to extend to the vicinity of an anode terminal 53 and thus the distance of a current path formed by the anode terminal 53, an anode lead 52, a capacitor element 51, and the cathode terminal 55 can be shortened with respect to the external circuit board 62, thereby enabling a further reduction in ESL. However, there is a problem that since a difference in area between a cathode exposed portion and an anode exposed portion increases, the solder 66 applied on the land 67 corresponding to a cathode terminal exposed portion 56 and thus having a larger area shrinks due to surface tension, so that the solid electrolytic capacitor with face-down terminals mainly placed on this solder 66 is raised and subjected to position offset, thus resulting in occurrence of failure in appearance and, further, occurrence of connection failure on the anode terminal side.
As a technique disclosed as means for solving this problem, there is an example of Patent Document 3 (Japanese Unexamined Patent Application Publication (JP-A) No. 2004-349270). In this example, the shape as shown in FIGS. 13A to 13C and FIG. 14 is proposed as that of a solid electrolytic capacitor with face-down terminals which can be well soldered to a circuit board or the like. FIGS. 13A to 13C and FIG. 14 show a solid electrolytic capacitor with face-down terminals according to prior art 3, wherein FIG. 13A is a front view thereof, FIG. 13B is a bottom view thereof, FIG. 13C is a right side view thereof, and FIG. 14 is a longitudinal sectional view showing the state where the capacitor is mounted on a circuit board.
At the bottom surface of the solid electrolytic capacitor with face-down terminals, a cathode terminal 55 has a first cathode exposed portion 58 exposed at a position near an anode exposed portion 54 where an anode terminal 53 is exposed, and a second cathode exposed portion 57 exposed at a position symmetrical to that of the anode exposed portion 54. As shown in FIG. 14, the cathode terminal 55 is provided with a concave portion by sputtering or the like between the first cathode exposed portion 58 and the second cathode exposed portion 57 and a casing resin is filled in the concave portion, thereby forming a cathode embedded portion 68.
The anode exposed portion 54 and the second cathode exposed portion 57 respectively extend to opposite ends (left and right ends in FIG. 13B) of the solid electrolytic capacitor with face-down terminals in a draw-out direction (implant direction) of an anode lead 52 of a capacitor element 51. On the other hand, the first cathode exposed portion 58 has extending portions 59 respectively extending to opposite ends (upper and lower ends in FIG. 13B) in a direction perpendicular to the draw-out direction of the anode lead 52, using the bottom surface of the solid electrolytic capacitor with face-down terminals as a reference.
However, in the solid electrolytic capacitor with face-down terminals in which the terminal shapes exposed at the bottom surface are not completely symmetrical on the left and right sides, when fixing it to a circuit board 62 through a solder 66, there is a possibility that a difference in surface tension occurs between the left and right sides during contraction of the solder and hence the capacitor is not accurately fixed to lands 67 on the circuit board 62, resulting in position offset and thus in occurrence of failure in appearance. Further, since the first cathode exposed portion 58 has the extending portions 59, the capacitor has a structure in which terminal cut surfaces are exposed on its four sides and, therefore, all the four sides should be cut into predetermined shapes by dicing or the like, leading to an increase in the number of processing steps and thus to an increase in production cost of products.
The foregoing prior art problems are summarized as follows. In the prior art of Patent Document 1, although the exposed terminals are symmetrical on the left and right sides, the current path distance between the anode and the cathode becomes long and thus the ESR and ESL of the entire capacitor increase at the high frequency range. In the prior art of Patent Document 2, since the area of the cathode exposed portion is greater than that of the anode exposed portion, the anode side is raised during contraction of the solder and thus the connection failure tends to occur. According to the prior art of Patent Document 3, in the solid electrolytic capacitor with face-down terminals in which the terminal shapes exposed at the mount surface are not completely symmetrical on the left and right sides, when fixing it to the circuit board through the solder, there is a possibility that a difference in surface tension occurs between the left and right sides during contraction of the solder and hence the capacitor is not accurately fixed to the lands on the circuit board, resulting in position offset and thus in occurrence of failure in appearance. Further, since the first cathode exposed portion has the extending portions, the capacitor has the structure in which the terminal cut surfaces are exposed on its four sides and, therefore, all the four sides should be cut into the predetermined shapes by dicing or the like, leading to an increase in the number of processing steps and thus to an increase in production cost of products.